I've personally replaced about half the windows in my home with new ones (See How to Install a Sash Replacement Window) but haven't yet thrown away the old ones. What to do with old windows? Use them in the garden!

I spotted these windows and doors in my neighborhood while on a walk. Some windows are being used for starting seeds, others as the tops for a mini greenhouse, and one old screen door as an entry way into a simple gated garden. I've been tempted to put mine to use but haven't yet had the need.